(1) Field of the Invention
The present invention relates to a sintered substrate of alkaline batteries and the manufacturing method thereof.
(2) Related Art
As substrate grids for electrodes of alkaline batteries including nickel-hydrogen storage batteries, sintered substrates have been commonly used. A sintered substrate is formed by applying nickel powder to a porous substrate made of nickel or the like, and by sintering the substrate. Among such porous substrates are punched metals and punched nickel plates. By impregnating the sintered substrate with an active material, an electrode can be formed. The production of alkaline batteries having a higher energy density, however, requires an increased amount of impregnated active material, since the operation of a battery is greatly dependent on the active material. To meet this requirement, it is preferable that sintered substrates have a high porosity and hold a sufficient amount of active material in those pores.
Such porous sintered substrate is attainable by using a punched metal with a reduced thickness or with an increased porosity. Though such methods are considered as valid, some other methods have been developed.
One example is a technique disclosed by the Japanese Patent Laid-Open Application Number S60-65464, in which a sintered substrate is formed by applying slurry made of nickel powder, water and pore former (hollow resinous particles) to a punched metal. Another example is a technique disclosed by the Japanese Patent Laid-Open Application Number S61-185685 in which a sintered substrate is formed by applying a mixture of water and metal-coated pore former particles to a punched metal and then sintering the punched metal together with the mixture.
FIG. 3 shows an electrode that uses a sintered substrate formed based on the former technique. Here, the nickel particles (FIG. 3B) and the core (punched metal) are sintered and bonded together to form a nickel framework (FIG. 3A). There are relatively small spaces among nickel particles in the nickel framework, and there are relatively large spaces between the nickel frameworks, which are formed by the effect of pore former
FIG. 4 shows an electrode that uses a sintered substrate manufactured according to the latter technique. As shown in the drawing, a punched metal and hollow spheric nickel shells (FIG. 4B) are sintered together to form a nickel framework (FIG. 4A). The nickel shells of this nickel framework (FIG. 4C) have relatively large pores, and they are joined together with some spaces in-between. Though not illustrated here for the sake of convenience, these pores and spaces are filled with an active material.
These two techniques can produce sintered substrates with relatively high porosity. However, the sintered substrates have a problem of strength. Sintered substrates manufactured by the former technique have a higher porosity, with relatively large pores formed inside the nickel frameworks by the effect of the pore former. But this porous construction can cause brittle failure, including surface detachment and cracking. Likewise, sintered substrates according to the second technique have pores of an average size generated from metal-plated pore former. But these sintered substrates do not have a sufficient level of strength, and they are weak especially against stress applied from the direction of the thickness. This can cause a problem similar to those with the first technique. Such problems can occur whether the shape of the battery is cylindrical or rectangular.